Flexible printed circuit board for optical module

ABSTRACT

A flexible printed circuit board (FPCB) for an optical module includes: a signal via pad connected with a signal lead pin of the optical module; a ground layer spaced apart from the signal via pad; an isolation gap formed between the signal via pad and the ground layer; and a protective layer which is formed at a portion that comprises the isolation gap, and which, when connected with the signal via pad, compensates for parasitic inductance caused by a protruding signal lead pin.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority from Korean Patent Application No.10-2015-0131888, filed on Sep. 17, 2015, in the Korean IntellectualProperty Office, the entire disclosure of which is incorporated hereinby reference for all purposes.

BACKGROUND

1. Field

The following description relates to a flexible printed circuit boardfor an optical module used in optical communications.

2. Description of the Related Art

An optical transceiver is a module that receives an electric signal togenerate an optical signal, or a module that receives an optical signaland converts the received optical signal into an electric signal. Theoptical transceiver is positioned at the end of an optical transmissionsystem or a router and serves as an optical interface. Such opticaltransceiver includes an optical transmission module and an opticalreception module, in which with the increased amount of datatransmission, the optical transmission module and the optical receptionmodule, which are core components, are becoming faster and smaller. Theoptical transmission module and the optical reception module may bedesigned to support both a single wavelength and multi wavelengthsdepending on applications, and may be configured in a package of variousshapes.

SUMMARY

Provided is a flexible printed circuit board for optical modules thatmay extend an operating bandwidth.

In one general aspect, there is provided a flexible printed circuitboard (FPCB) for an optical module, the FPCB including: a signal via padconnected with a signal lead pin of the optical module; a ground layerspaced apart from the signal via pad; an isolation gap formed betweenthe signal via pad and the ground layer; and a protective layer which isformed at a portion that comprises the isolation gap, and which, whenconnected with the signal via pad, compensates for parasitic inductancecaused by a protruding signal lead pin.

The protective layer may induce a capacitance component in the isolationgap to compensate for the parasitic inductance caused by the protrudingsignal lead pin.

The protective layer may have a higher dielectric constant than air. Theprotective layer may be a cover layer filled with a coating material.The protective layer may be filled with a dielectric material, and thedielectric material may be a bonding material.

The protective layer may include: a first protective layer formed at aportion that comprises a first isolation gap provided between the signalvia pad and a top-side ground layer; and a second protective layerformed at a portion that comprises a second isolation gap providedbetween the signal via pad and a bottom-side ground layer. The firstisolation gap and the second isolation gap may be identical to ordifferent from each other.

The first protective layer may be a first cover layer filled with afirst coating material; and the second protective layer may be a secondcover layer filled with a second coating material, wherein the firstcoating material and the second coating material may be identical to ordifferent from each other.

The first protective layer may be filled with a first dielectricmaterial; and the second protective layer is filled with a seconddielectric material, wherein the first dielectric material and thesecond dielectric material may be identical to or different from eachother.

In another general aspect, there is provided a flexible printed circuitboard (FPCB) for an optical module, the FPCB including: a top-side coverlayer formed on the top of the FPCB; a top-side ground layer formed onthe bottom of the top-side cover layer; a bottom-side ground layerconnected with the top-side ground layer through a ground via; a signalvia pad having an upper portion spaced apart from the top-side groundlayer, and a lower portion spaced apart from the bottom-side groundlayer; a bottom-side cover layer formed on the bottom of the bottom-sideground layer; a first isolation gap formed between the top-side groundlayer and the upper portion of the signal via pad; and a secondisolation gap formed between the bottom-side ground layer and the lowerportion of the signal via pad, wherein either one of the first isolationgap and the second isolation gap is filled with a coating material or adielectric material.

The first isolation gap may be filled with a same material as thetop-side cover layer; and the second isolation gap may be filled with asame material as the bottom-side cover layer. The first isolation gapmay be filled with a top-side dielectric material; and the secondisolation gap may be filled with a bottom-side dielectric material. Thefirst isolation gap may be filled with air; and the second isolation gapmay be filled with the bottom-side dielectric material. The firstisolation gap may be filled with a same material as the top-side coverlayer; and the second isolation gap may be filled with the bottom-sidedielectric material. The first isolation gap may be filled with air; andthe second isolation gap may be filled with a same material as thebottom-side cover layer. The first isolation gap may be filled with asame material as the top-side cover layer; and the second isolation gapmay be filled with air. The first isolation gap may be filled with thetop-side dielectric material; and the second isolation gap may be filledwith air.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an optical reception module accordingto an exemplary embodiment.

FIG. 2 is a plan view illustrating a flexible printed circuit board(“FPCB”) seen from the left of FIG. 1.

FIG. 3 is a diagram illustrating an FPCB according to a first exemplaryembodiment.

FIG. 4 is a diagram illustrating an FPCB according to a second exemplaryembodiment.

FIG. 5 is a diagram illustrating an FPCB according to a third exemplaryembodiment.

FIG. 6 is a diagram illustrating an FPCB according to a fourth exemplaryembodiment.

FIG. 7 is a graph illustrating a return loss according to a frequency ofan FPCB according to an exemplary embodiment.

FIG. 8 is a graph illustrating a return loss according to a frequency ofan FPCB according to another exemplary embodiment.

DETAILED DESCRIPTION

Hereinafter, exemplary embodiments of the present disclosure will bedescribed in detail with reference to the accompanying drawings. In thefollowing description, a detailed description of known functions andconfigurations incorporated herein will be omitted when it may obscurethe subject matter of the present invention. Further, the terms usedthroughout this specification are defined in consideration of functionsaccording to exemplary embodiments, and can be varied according to apurpose of a user or manager, or precedent and so on. Therefore,definitions of the terms should be made on the basis of the overallcontext.

FIG. 1 is a diagram illustrating an optical reception module accordingto an exemplary embodiment.

More specifically, FIG. 1 is a diagram illustrating an optical receptionmodule that is based on a transistor outline (TO) package and maysupport a single wavelength. An optical transmission module may beconfigured to have the same shape as the optical reception module. Inaddition to the TO package, the optical reception module may also have ashape of a box, e.g., a square box.

The optical reception module is largely composed of an optical module 1and a flexible printed circuit board (“FPCB”) 2. The optical module 1receives an optical signal, converts the received optical signal into anelectric signal, and outputs the electric signal. The output electricsignal is transmitted to a main PCB board through the FPCB 2. Theoptical module 1 and the FPCB 2 are connected with each other so thatthe electric signal is transmitted through a path of 90 degrees, asshown by the reference numeral 100 in FIG. 1, and the optical module 1and the FPCB 2 are fixed by soldering as shown by the reference numeral110 in FIG. 1. The reference numeral 120 shows a portion of the FPCBconnected to the main PCB board.

FIG. 2 is a plan view illustrating a FPCB seen from the left of FIG. 1.

Referring to FIG. 2, the FPCB 2 may be divided into a portion 200connected to an optical module and a portion 210 connected to a main PCBboard, in which the main PCB board may be a main board in an opticaltransmission/reception section or an optical transceiver.

A substrate base 290 of the FPCB 2 is flexible enough to be readilybent. A signal via pad 230 includes an upper signal pad and a lowersignal pad that are disposed on the same axis, in which the upper signalpad and the lower signal pad may be connected with each other through asignal via. The signal via pad 230 has a through hole formed at aportion corresponding to a signal via. The through hole has a diameterlarge enough for a signal lead pin to be inserted through the throughhole, and is formed to vertically pass through the center of the signalvia. With the signal lead pin of the optical module being inserted intothe through hole, when the signal lead pin is soldered to the lowersignal pad, the signal lead pin is connected to the upper signal pad aswell as the lower signal pad through the signal via, and thus isconnected to a signal line 250. Although there are two signal via pads230 in FIG. 2, the number is not limited thereto.

The signal line 250 is disposed on the top of the substrate base 290.The signal line 250 extends from the upper signal pad along alongitudinal direction of the substrate base 290. The signal line 250may receive a high-speed signal, and may be formed of a conductivematerial and formed to have a uniform width and thickness. The signalline 250 may be a data signal line for transmitting a data signal. Aconnection pad 270 connectable to a main PCB board may be disposed at arear end of the signal line 250.

Driving signal lines 240 for transmitting a power signal or othersignals for monitoring/control may be provided on the top of thesubstrate base 290. A signal pad 220, having the same shape as thesignal via pad 230, is disposed at a front end portion of each of thedriving signal lines 240, and thus is connected to a driving signal leadpin of the optical communication module. A connection pad 260connectable to the main PCB board may be disposed at a rear end portionof the driving signal line 240. A ground pad 280 may be interposedbetween the connection pad 270 connected to the signal line 250 and theconnection pad 260 connected to each of the driving signal lines 240.

Hereinafter, the structure of the FPCB taken in line A-A′ of FIG. 2 willbe described in detail with reference to FIGS. 3 to 6. Referring toFIGS. 3 to 6, by providing an isolation gap between the signal via padand a ground layer included in the FPCB, and by filling the isolationgap with a protective layer, parasitic inductance caused by the signallead pin of the optical module may be compensated for.

The protective layer in a first exemplary embodiment may be abottom-side cover layer 370 a as illustrated in FIG. 3. The protectivelayer in a second exemplary embodiment may include a bottom-sidedielectric material 400 as illustrated in FIG. 4. The protective layerin a third exemplary embodiment may include a top-side cover layer 340 band a bottom-side cover layer 370 c as illustrated in FIG. 5, in whichthe top-side cover layer 340 b and the bottom-side cover layer 370 c maybe formed of the same material or different materials. The protectivelayer in a fourth exemplary embodiment may include a bottom-sidedielectric material 600 and a top-side dielectric material 610 asillustrated in FIG. 6.

FIG. 3 is a diagram illustrating an FPCB according to a first exemplaryembodiment.

Referring to FIG. 3, the FPCB 2 a includes a signal via pad 310, a corelayer 330 a, a top-side cover layer 340 a, a bottom-side ground layer350 a, an isolation gap 360 a, and a bottom-side cover layer 370 a.

When the signal lead pin 10 of the optical module is inserted throughthe through hole of the FPCB 2 a, the signal lead pin 10 is cut toslightly protrude 300, and is soldered with solder 320 to the signal viapad 310 and to thereby be fixed therewith. In this case, the protrudinglead pin 300 unintentionally causes parasitic inductance. In order tocompensate for the parasitic inductance, the isolation gap 360 a isprovided between the bottom-side ground layer 350 a and the signal viapad 310, to induce a parasitic capacitance component.

The isolation gap 360 a is generally exposed to the air. In this case,due to parasitic capacitance between the signal via pad 310 and thebottom-side ground layer 350 a, it is difficult to compensate for aparasitic inductance component resulting from the protruding lead pin300, thereby limiting a manufacturing process of the FPCB. The isolationgap, which is provided between the bottom-side ground layer 350 a andthe signal via pad 310 to induce a desired parasitic capacitancecomponent, may be limited depending on conditions of manufacturing theFPCB, and there may be a case where the gap may not be reduced withoutlimitation.

Referring to FIG. 3, the isolation gap 360 a, which is provided betweenthe bottom-side ground layer 350 a and the signal via pad 310 to inducea desired parasitic capacitance component, is filled with thebottom-side cover layer 370 a instead of air. The bottom-side coverlayer 370 a, having a higher dielectric constant than air, providesprotection and electrical insulation, to induce parasitic capacitance. Acoating material generally used for the manufacture of the FPCB may beused as the bottom-side cover layer 370 a. Bottom-side cover layer 370 acovers a bottom side of a lower portion of signal via pad 310 and coversbottom-side ground layer 350 a.

The core layer 330 a of the FPCB 2 a may include a polyimide-basedmaterial, a Teflon-based material, a material obtained by combining apolyimide-based material and a Teflon-based material, and a dielectricmaterial having flexibility.

FIG. 4 is a diagram illustrating an FPCB according to a second exemplaryembodiment.

Referring to FIG. 4, the FPCB 2 b includes a signal via pad 310, a corelayer 330 a, a top-side cover layer 340 a, a bottom-side ground layer350 a, an isolation gap 360 a, and a bottom-side cover layer 370 bincluding a bottom-side dielectric material 400. Bottom-side cover layer370 b covers a bottom side of a lower portion of signal via pad 310,covers a bottom side of bottom-side ground layer 350 a and fillsisolation gap 360 a. Dielectric material 400 of bottom-side cover layer370 b fills isolation gap 360 a, covers the bottom side of the lowerportion of signal via pad 310 and covers a portion of the bottom side ofbottom-side ground layer 350 a.

In the FPCB 2 b, instead of filling the isolation gap 360 a with thebottom-side cover layer 370 b, the isolation gap 360 a between thesignal via pad 310 and the bottom-side ground layer 350 a is filled withthe bottom-side dielectric material 400 having a higher dielectricconstant than the bottom-side cover layer 370 b. The bottom-sidedielectric material 400 may be a bonding material, for example, epoxy,which may enable a firm physical connection of the optical module andthe FPCB 2 b.

The top-side cover layer 340 a and the bottom-side cover layer 370 b,which are used in the FPCB 2 b, may be made of the same material ordifferent materials. Further, the FPCB 2 b may be composed of aplurality of layers configured in a stack.

FIG. 5 is a diagram illustrating an FPCB according to a third exemplaryembodiment.

Referring to FIG. 5, the FPCB 2 c includes a signal via pad 310, a corelayer 330 b, a top-side cover layer 340 b, a bottom-side ground layer350 b, a first isolation gap 1 360 b, a second isolation gap 2 360 c, abottom-side cover layer 370 c, a top-side ground layer 500 a, and aground via 510 a.

When the signal lead pin 10 of the optical module is inserted throughthe through hole of the FPCB 2 c, the signal lead pin 10 is cut toslightly protrude 300, and is soldered. In order to compensate for aparasitic inductance component caused by the protruding lead pin 300,the first isolation gap 360 b is provided between the top-side groundlayer 500 a and the signal via pad 310 of the FPCB 2 c, and the secondisolation gap 360 c is provided between the bottom-side ground layer 350b and the signal via pad 310, so as to induce a parasitic capacitancecomponent. In this case, the first isolation gap 360 b and the secondisolation gap 360 c, which are provided to induce a desired capacitancecomponent, may be limited depending on conditions of manufacturing theFPCB, and there may be a case where the gap may not be reduced withoutlimitation.

Referring to FIG. 5, instead of filling the first isolation gap 360 bwith air, the first isolation gap 360 b is filled with the top-sidecover layer 340 b generally used for protection and electricalinsulation when manufacturing the FPCB. Further, instead of filling thesecond isolation gap 360 c with air, the second isolation gap 360 c isfilled with the bottom-side cover layer 370 c generally used forprotection and electrical insulation when manufacturing the FPCB. Thetop-side cover layer 340 b and the bottom-side cover layer 370 c have ahigher dielectric constant than air.

The top-side cover layer 340 b and the bottom-side cover layer 370 c maybe made of the same material or different materials. Further, the FPCB 2c may be composed of a plurality of layers configured in a stack. Thefirst isolation gap 360 b and the second isolation gap 360 c may bedifferent from each other. The ground via 510 a connects the top-sideground layer 500 a and the bottom-side ground layer 350 b, and may bemade of a conductive material.

The ground via 510 a electrically connects the top-side ground layer 500a and the bottom-side ground layer 350 b.

FIG. 6 is a diagram illustrating an FPCB according to a fourth exemplaryembodiment.

Referring to FIG. 6, the FPCB 2 d includes a signal via pad 310, a corelayer 330 b, a top-side cover layer 340 c, a bottom-side ground layer350 c, a first isolation gap 1 360 d, a second isolation gap 2 360 e, abottom-side cover layer 370 d, a top-side ground layer 500 b, a groundvia 510 b, a bottom-side dielectric material 600, and a top-sidedielectric material 610.

In FIG. 6, the FPCB 2 d includes: the top-side cover layer 340 c formedon the top of the FPCB 2 d; the top-side ground layer 500 b formed onthe bottom of the top-side cover layer 340 c; the bottom-side groundlayer 350 c connected with the top-side ground layer 500 b through theground via 510 b; the signal via pad 310 having an upper portion spacedapart from the top-side ground layer 500 b, and a lower portion spacedapart from the bottom-side ground layer 350 c; the bottom-side coverlayer 370 d formed on the bottom of the bottom-side ground layer 350 c;the first isolation gap 360 d interposed between the top-side groundlayer 500 b and the upper portion of the signal via pad 310; and thesecond isolation gap 360 e interposed between the bottom-side groundlayer 350 c and the lower portion of the signal via pad 310.

In the FPCB 2 d, the first isolation gap 360 d is provided between thetop-side ground layer 500 b and an upper portion of the signal via pad310; and the second isolation gap 360 e is provided between thebottom-side ground layer 350 c and a lower portion of the signal via pad310. In this case, instead of filling the first isolation gap 360 d withthe top-side cover layer 340 c, the first isolation gap 360 d is filledwith the top-side dielectric material 610 having a higher dielectricconstant than a material of the top-side cover layer 340 c. Further,instead of filling the second isolation gap 360 e with the bottom-sidecover layer 370 d, the second isolation gap 360 e is filled with thebottom-side dielectric material 600 having a higher dielectric constantthan a material of the bottom-side cover layer 370 d. The dielectricmaterial may be a dielectric function material, or may be a bondingmaterial, such as epoxy, used for a firm physical connection between theoptical module and the FPCB 2 d. The bottom-side dielectric material 600and the top-side dielectric material 610 may be configured in differentmanners from each other.

In the FPCB, the first isolation gap 360 d and the second isolation gap360 e may be configured in different manners as follows. For example,the first isolation gap 360 d may be filled with air, and the secondisolation gap 360 e may be filled with the bottom-side dielectricmaterial 600. Alternatively, the first isolation gap 360 d may be filledwith the same material as the material of the top-side cover layer 340c, and the second isolation gap 360 e may be filled with the bottom-sidedielectric material 600. In addition, the first isolation gap 360 d maybe filled with air, and the second isolation gap 360 e may be filledwith the same material as the material of the bottom-side cover layer370 d. Moreover, the first isolation gap 360 d may be filled with thesame material as the material of the top-side cover layer 340 c, and thesecond isolation gap 360 e may be filled with air. Further, the firstisolation gap 360 d may be filled with the top-side dielectric material610, and the second isolation gap 360 e may be filled with air. In theabove examples, either one of the first isolation gap 360 d and thesecond isolation gap 360 e is filled with a material of the cover layeror a dielectric material. However, such examples are merelyillustrative, and the first and the second isolation gaps may beconfigured in various other manners.

FIG. 7 is a graph illustrating a return loss according to a frequency ofan FPCB according to an exemplary embodiment.

Referring to FIG. 7, return loss values may be obtained for thefollowing cases: a case where an isolation gap between the bottom-sideground layer and the signal via pad is filled with air (dielectricconstant=1) in 700; a case where an isolation gap between thebottom-side ground layer and the signal via pad is filled with thebottom-side cover layer (dielectric constant=3.4) in 710; and a casewhere an isolation gap between the bottom-side ground layer and thesignal via pad is filled with a bonding material (epoxy with adielectric constant=6) in 720. The dielectric material (fillingmaterial) used in the calculation is merely an example for theconvenience of explanation, and the present disclosure is not limitedthereto. By adjusting resonance points on the return loss curveaccording to filling materials having different dielectric constants, areturn loss may be induced to a specific value in a desired frequencyrange or less.

FIG. 8 is a graph illustrating a return loss according to a frequency ofan FPCB according to another exemplary embodiment.

Referring to FIG. 8, return loss values may be obtained for thefollowing cases: a case where a first isolation gap 1 between thetop-side ground layer and the signal via pad and a second isolation gap2 between the bottom-side ground layer and the signal via pad are filledwith air (dielectric constant=1) in 800; a case where the firstisolation gap 1 and the second isolation gap 2 are filled with thetop-side cover layer (dielectric constant=3.4) and the bottom-side coverlayer (dielectric constant=3.4) respectively in 810; and a case wherethe first isolation gap 1 and the second isolation gap 2 are filled witha bonding material (epoxy with a dielectric constant=6) in 820.

Referring to FIG. 8, it can be seen that the same material was used as abonding material in 820 to fill the first isolation gap 1 and the secondisolation gap 2. The dielectric material (filling material) used in thecalculation is merely an example for the convenience of explanation, andthe present disclosure is not limited thereto. By adjusting resonancepoints on the return loss curve according to filling materials havingdifferent dielectric constants, a return loss may be induced to aspecific value in a desired frequency range or less.

According to the present disclosure, when a signal lead pin of anoptical module is connected with a flexible printed circuit board(“FPCB”), the signal lead pin is cut to protrude, thereby compensatingfor unintentional parasitic inductance. That is, by providing anisolation gap between a signal via pad of the FPCB and a ground layerthat surrounds the FPCB, and by filling the isolation gap with a coatingmaterial used for manufacturing the FPCB or a dielectric material havinga dielectric constant, a parasitic inductance component may becompensated for, and a desired capacitance component may be readilyinduced. Further, By using a bonding material, such as epoxy, as adielectric material, a firm connection between the optical module andthe FPCB may be maintained.

A number of examples have been described above. Nevertheless, it shouldbe understood that various modifications may be made. For example,suitable results may be achieved if the described techniques areperformed in a different order and/or if components in a describedsystem, architecture, device, or circuit are combined in a differentmanner and/or replaced or supplemented by other components or theirequivalents. Accordingly, other implementations are within the scope ofthe following claims. Further, the above-described examples are forillustrative explanation of the present invention, and thus, the presentinvention is not limited thereto.

What is claimed is:
 1. A flexible printed circuit board (FPCB) for anoptical module, the FPCB comprising: a signal via pad connected with asignal lead pin of the optical module; a bottom-side ground layer spacedapart from a lower portion of the signal via pad; a first isolation gapformed between lower portion of the signal via pad and the bottom-sideground layer; and a protective layer comprising a first cover layercovering a bottom side of the lower portion of the signal via pad,covering a bottom side of the bottom-side ground layer, filling thefirst isolation gap, and that compensates for, parasitic inductancecaused by a protruding signal lead pin.
 2. The FPCB of claim 1, whereinthe protective layer induces a capacitance component in the firstisolation gap to compensate for the parasitic inductance caused by theprotruding signal lead pin.
 3. The FPCB of claim 1, wherein theprotective layer has a higher dielectric constant than air.
 4. The FPCBof claim 1, wherein the protective layer is a cover layer filled with acoating material.
 5. The FPCB of claim 1, wherein the protective layeris filled with a dielectric material.
 6. The FPCB of claim 1, whereinthe dielectric material is a bonding material.
 7. The FPCB of claim 1,wherein the FPCB comprises: a top-side ground layer spaced apart from anupper portion of the signal via pad and a second isolation gap formedbetween the upper portion of the signal via pad and the top-side groundlayer, and wherein the protective layer comprises: a second cover layercovering a top side of the upper portion of the signal via pad, coveringa top side of the top-side ground layer, and filling the secondisolation gap.
 8. The FPCB of claim 7, wherein the first isolation gapand the second isolation gap are identical to or different from eachother.
 9. The FPCB of claim 7, wherein: the first cover layer comprisesa first coating material; and the second cover layer comprises a secondcoating material, wherein the first coating material and the secondcoating material are identical to or different from each other.
 10. TheFPCB of claim 7, wherein: the first cover layer is filled with a firstdielectric material; and the second cover layer is filled with a seconddielectric material, wherein the first dielectric material and thesecond dielectric material are identical to or different from eachother.